Allow using GTK+ to handle input methods on X

Allow using GTK+ to handle input methods on X

* doc/emacs/xresources.texi (Table of Resources): Document new
value of `inputStyle'.
* etc/NEWS: Announce new option.

* lisp/cus-start.el (standard): Add `x-gtk-use-native-input'.
* src/gtkutil.c (xg_mark_data): Mark xg_pending_quit_event.
(xg_add_virtual_mods):
(xg_im_context_commit):
(xg_im_context_preedit_changed):
(xg_im_context_preedit_end):
(xg_widget_key_press_event_cb):
(xg_filter_key): New functions.
* src/gtkutil.h: Add prototype for `xg_filter_key'.
* src/xfns.c (xic_set_preeditarea): Set cursor location for the
GTK IM context as well.
* src/xterm.c (xg_pending_quit_event): New variable.
(x_focus_changed): Set focus on the GTK input context as well.
(x_filter_event): Filter events through GTK if the user asked
for it.
(handle_one_xevent): Likewise.
(XTread_socket): Set hold_quit to xg_pending_quit_event if it
exists.
(x_draw_window_cursor): Always set preedit area even if XIC
doesn't exist.
* src/xterm.h (struct x_display_info): New field
`prefer_native_input'.
(struct x_output): New field `im_context'.